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Deciphering western Mediterranean kinematics using metamorphic porphyroblasts from the Alpujárride Complex (Betic Cordillera)

February 2023
Journal of Structural Geology 168(4):104823

DOI:10.1016/j.jsg.2023.104823

License
CC BY-NC-ND 4.0

Authors:

Alejandro Ruiz-Fuentes

Domingo Aerden

University of Granada

3D microstructural analysis of porphyroblast inclusion trails using X-ray Computed Tomography is integrated with analysis of field structures to unravel the Alpine deformation history of the Alpuj´arride Complex, which constitutes the partially submerged metamorphic core of the Gibraltar Arc. Prograde metamorphism in the complex has been traditionally linked to a ’D1’ event witnessed by inclusion trails in garnet porphyroblasts. Orientation data for these microstructures reveal three age groups with differently oriented axes of inclusion-trail curvature (known as FIA). The successive development of FIAs trending WNW-ESE, ENE-WSW and NNW-SSE is shown and correlated with the Paleogene-Neogene relative plate-motion paths of Africa, Iberia and the Albor´an Domain as known from paleomagnetic data. During the late-metamorphic evolution of the Alpuj´arride Complex, after garnet growth had ceased, two steeply dipping crenulation cleavages and associated folds with roughly suborthogonal N–S and E-W trends developed, in addition to two subhorizontal ones. Inclusion trails are also found to exhibit a general preference for subvertical and subhorizontal orientations, suggesting a protracted orogenic evolution characterized by multiple stress permutations causing alternations of crustal shortening and gravitational collapse.

Geological map of the Internal Zones of the Betic Cordillera, which as the more detailed maps of Figs. 2 and 3 are based on IGME MAGNA 1/50 000 map series.

…

Geological maps of the Tocón de Quéntar (A) and Contraviesa (B) areas showing the location of cross sections, outcrop and microstructural images (Figure numbers in small boxes), average microstructural trends indicated with red, orange and green bars corresponding to the three trend ranges defined in Fig. 11, and the directions of tectonic transport (thin arrows) deduced from inclusion-trail asymmetries. Map 'A' is partially based on Sanz de Galdeano et al. (1995). (For interpretation of the references to color in this figure legend, the reader is referred to the Web version of this article.)

…

Outcrop images of S 3H . A) Intensely developed S 3H in quartzites situated near the middle of the Jubrique schist-quartzite formation. B) Widely spaced shear bands in the lower levels of the same formation. C) F 3H folds in gneisses of the Jubrique Fm. D) Folds associated with S 3H in the Benalmádena area. E) S 3H near Torrox transposing S 2 in a pelitic layer above a less deformed psammitic layer showing F 3H folds. F) Interference of F 2 and F 3H folds near La Herradura. G) Coeval D 3H folds and shear bands in the Contraviesa area. H) Subhorizontal shear bands south of Albondón genetically related to S 3H .

…

Outcrop images of S 3V . A) S 3V is intensely developed and becomes the main foliation in outcrop south of Mijas. Fig. 10a shows a microscope image of the S 3V crenulations. B) Intensely folded gneisses south of Mijas. C) S 3V overprinting D 3H folds east of Almuñécar. Fig. 10b shows a microscopic image of the same outcrop. D) Fold interference in amphibolites of the Herradura nappe west of Almuñécar with S 3V overprinting S 3H , in turn overprinting an F 2 fold. Fig. 10h shows a microscope image of the same outcrop. E) South-vergent D 3V folds north of Tocón de Quéntar. F) N-S L 2 overprinted by E-W F 3V folds north of Tocón de Quéntar. G) D 3V folds cut by D 3H shear bands in the Contraviesa area. H) S 3V partially transposing S 2 in the Contraviesa area.

…

Equal-area and lower-hemisphere projections for: total porphyroblast FIAs measured in all areas, all fold axes and lineations shown separately for different areas in Fig. 7, and a similar compilation of poles to all crenulation cleavages shown in Fig. 7. All equal-area projections were made with the program 'Stereonet' by Allmendinger et al. (2013) and use Kamb contours with contour interval and significance level set to 2 or 3σ (see figure).

…

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Journal of Structural Geology 168 (2023) 104823

Available online 9 February 2023

nc-nd/4.0/).

Deciphering western Mediterranean kinematics using metamorphic

porphyroblasts from the Alpuj´

arride Complex (Betic Cordillera)

Alejandro Ruiz-Fuentes

, Domingo G.A.M. Aerden

Departamento de Geodin´

amica, Universidad de Granada, Av. Fuentenueva, 18071, Granada, Spain

Instituto Andaluz de Ciencias de la Tierra, CSIC/Universidad de Granada, Av. de las Palmeras 4, 18100, Armilla, Granada, Spain

ARTICLE INFO

Keywords:

Alpuj´

arride complex

Betic cordillera

Alpine orogeny

Foliation intersection axes

X-ray computed tomography

ABSTRACT

3D microstructural analysis of porphyroblast inclusion trails using X-ray Computed Tomography is integrated

with analysis of eld structures to unravel the Alpine deformation history of the Alpuj´

arride Complex, which

constitutes the partially submerged metamorphic core of the Gibraltar Arc. Prograde metamorphism in the

complex has been traditionally linked to a ’D

’ event witnessed by inclusion trails in garnet porphyroblasts.

Orientation data for these microstructures reveal three age groups with differently oriented axes of inclusion-trail

curvature (known as FIA). The successive development of FIAs trending WNW-ESE, ENE-WSW and NNW-SSE is

shown and correlated with the Paleogene-Neogene relative plate-motion paths of Africa, Iberia and the Albor´

Domain as known from paleomagnetic data. During the late-metamorphic evolution of the Alpuj´

arride Complex,

after garnet growth had ceased, two steeply dipping crenulation cleavages and associated folds with roughly

suborthogonal N–S and E-W trends developed, in addition to two subhorizontal ones. Inclusion trails are also

found to exhibit a general preference for subvertical and subhorizontal orientations, suggesting a protracted

orogenic evolution characterized by multiple stress permutations causing alternations of crustal shortening and

gravitational collapse.

1. Introduction

In the Alpuj´

arride Complex of the Betic Cordillera, a succession of at

least three tectonic foliations and associated fold sets has been recog-

nized but no consensus has been reached on their tectonic signicance

(extension, compression, transpression, etc.) and kinematics (see review

in e.g. Williams and Platt, 2017; Jabaloy-S´

anchez et al., 2019a). These

structures are predated by one or several foliations that are only pre-

served as inclusion trails within garnet porphyroblasts. The latter grew

during prograde metamorphism, whereas the main external fabrics

formed during decompression. This study focuses on the geometry and

preferred orientations of these relic microstructures using oriented thin

sections and X-ray computed microtomography (XCT) in 60 samples

that span about 200 km distance in E-W direction. This microstructural

work is integrated with conventional analysis of structural relationships

in outcrop along various transects.

The principle difculty for interpreting the tectonic signicance of

relic foliations has traditionally been that their original orientation was

uncertain. In recent decades, however, remarkably consistent orienta-

tions of inclusion trails found in numerous mountain belts suggest that

they preserve the original orientations in which they formed due to a

general lack of porphyroblast rotation (e.g. Fyson, 1980; Bell et al.,

1992; Hayward, 1992; Aerden, 1995, 1998, 2004; Bell and Forde, 1995;

Bell and Welch, 2002; Stallard and Hickey, 2001; Bell et al., 2003; Ham

and Bell, 2004; Sayab, 2005; Cihan et al., 2006; Sanislav, 2010; Sanislav

and Bell, 2011; Shah et al., 2011; Skrzypek et al., 2011; Bell and Sap-

kota, 2012; Abu Sharib and Sanislav, 2013; Aerden et al., 2010, 2013,

2021, 2022; Sayab et al., 2016). Inclusion trail data in these studies have

consequently allowed extending tectonic reconstructions further back in

time and in more detail. Of particular interest are the orientations of so

called Foliation Intersection- or Inexion-Axes (Bell et al., 1995),

abbreviated as FIA (equivalent to relative porphyroblast-matrix rotation

axes) as they are thought to form normal to the direction of tectonic

transport or crustal shortening. Aerden et al. (2022) recently presented a

collection of ca. 350 FIA data from the Betic-Rif orogen and showed that

they cluster in three sets with distinctive trends (WNW-ESE, NE-SW, and

NNW-SSE). Relative timing criteria and Sm–Nd garnet dating supported

the sequential formation of the three FIA sets, from the Late Eocene to

the Early Miocene, oriented normal to (changing) relative plate-motion

vectors between Africa, Iberia and the Albor´

an Domain. Most of Aerden

* Corresponding author.

E-mail addresses: aruizf@ugr.es (A. Ruiz-Fuentes), aerden@ugr.es (D.G.A.M. Aerden).

Contents lists available at ScienceDirect

Journal of Structural Geology

journal homepage: www.elsevier.com/locate/jsg

https://doi.org/10.1016/j.jsg.2023.104823

Received 7 July 2022; Received in revised form 7 February 2023; Accepted 8 February 2023

Journal of Structural Geology 168 (2023) 104823

et al.’s data, however, were collected in the Nevado-Fil´

abride and Seb-

tide complexes with only 47 individual FIAs coming from the

Alpuj´

arride Complex, despite its much larger total outcrop area

compared to the other two complexes. In this paper, we present 647 new

individual plus 25 average FIAs for the Alpuj´

arride Complex and

document their relationships with polyphase structures and fabrics

recognizable in the eld. This evidence conrms the existence of three

FIA sets in the Betic-Rif orogen and supports their relationship with

orogen dynamics and the Tertiary paleogeographic evolution of the

Western Mediterranean region.

2. Geological setting

2.1. The Betic Cordillera

The Betic Cordillera (Fig. 1) of southern Spain is situated at the

western termination of the Mediterranean Alpine system and constitutes

the northern branch of the Gibraltar Arc. Its ’Internal Zones’ comprise a

stack of metamorphic complexes arranged, from bottom to top, as the

Nevado-Fil´

abride, Alpuj´

arride and Mal´

aguide complexes. The rst is

generally considered part of subducted and exhumed South-Iberian

crust (Platt et al., 2006; G´

omez-Pugnaire et al., 2012; Rodríguez-Ca-

nero et al., 2018), whereas the Alpuj´

arride and Mal´

aguide complexes

jointly constitute the so called ’Albor´

an Domain’ which constitutes the

core of the Gibraltar Arc and crops out also in the Rif mountains of

northern Morocco. The equivalent units of the Alpuj´

arride and

Mal´

aguide complexes are called Sebtide complex and Ghomaride com-

plex there. The Albor´

an domain is derived from a continental fragment

known as AlKaPeCa (Bouillin, 1986) or Mesomediterranean Microplate

(Guerrera et al., 1993) originally situated east or south-east of Iberia (see

review in Guerrera et al., 2021). Westward drift of the Albor´

an Domain,

independent of continuous N–S to NW-SE-changing Iberia-Africa

convergence, led to its eventual collision with the South Iberian paleo-

margin and the development of an external fold-and-thrust belt in

Mesozoic and Cenozoic sedimentary rocks of shelf and basin environ-

ments corresponding to the present External Zones and Flysch Units (or

Campo de Gibraltar Complex), respectively. Cover sediments of the

Albor´

an Domain appear at the front of the Internal Zones as several

tectonic slices known as the Frontal Units (or Dorsale Calcaire). These

are overthrusted by the Alpuj´

arride-Mal´

aguide stack, but also partially

backthrusted onto these complexes (Jabaloy-S´

anchez et al., 2019b).

Pre-Alpine structures in the Paleozoic basements of the three com-

plexes are only well preserved in the low-grade to non-metamorphic

Mal´

aguide Complex (e.g. Foucault and Paquet, 1971; Balany´

a, 1991;

Cuevas et al., 2001; Martín-Algarra et al., 2009; Ruiz-Fuentes et al.,

2022). In the Alpuj´

arride and Nevado-Fil´

abride complexes, their

recognition is complicated by intense Alpine deformation and meta-

morphism, but nevertheless evidenced by geochronological and petro-

logical data (e.g. Puga and Díaz de Federico, 1976; S´

anchez-Navas et al.,

2012, 2014, 2017; Acosta-Vigil et al., 2014; Massonne, 2014; Sanz de

Galdeano and Ruiz Cruz, 2016).

The Mal´

aguide Complex is only affected by Alpine metamorphism in

its lower part, which reached sub-greenschist facies conditions (Nieto

et al., 1994; Ruiz Cruz et al., 2005). The Alpuj´

arride Complex underwent

high-pressure/low-temperature (HP/LT) metamorphism in the Eocene

(Platt et al., 2005; Bessi`

ere et al., 2022) followed by progressive heating

during the Oligocene and exhumation in the Miocene (Aguado et al.,

1990; Crespo-Blanc et al., 1994; Sosson et al., 1998; S´

anchez-Rodríguez

and Gebauer, 2000; Esteban et al., 2004; Massonne, 2014). Exhumation

of the complex was synchronous with HP metamorphism in the Neva-

do-Fil´

abride Complex (L´

opez S´

anchez-Vizcaíno et al., 2001; Platt et al.,

2006; Kirchner et al., 2016), which was exhumed in the late Miocene

(Johnson et al., 1997). Recent geochronological and petrological evi-

dence, however, indicates that the Nevado-Fil´

abrides may have expe-

rienced an earlier HP metamorphic event in the Eocene, synchronous

with that of the Alpuj´

arride Complex (Augier et al., 2005; Li and Mas-

sonne, 2018; Porkol´

ab et al., 2022; Aerden et al., 2022).

2.2. Alpuj´

arride Complex

The Alpuj´

arride Complex has been traditionally subdivided in a large

number of nappe units dened and named in restricted study areas (e.g.

Los Reales nappe (Tubía, 1988), Sayalonga unit (Aldaya et al., 1979),

Murtas nappe (Aldaya, 1969), La Plata nappe (García-Due˜

nas and

Navarro-Vil´

a, 1976), etc.). Attempts to regionally correlate these units

resulted in partially discrepant proposals (see Sanz de Galdeano and

L´

opez-Garrido, 2003 for a review). In the Central and eastern Betics,

Aza˜

n´

on et al. (1994) distinguished ve units called, from bottom to top,

Fig. 1. Geological map of the Internal Zones of the Betic Cordillera, which as the more detailed maps of Figs. 2 and 3 are based on IGME MAGNA 1/50 000

map series.

A. Ruiz-Fuentes and D.G.A.M. Aerden

Journal of Structural Geology 168 (2023) 104823

the Lújar-, Escalate-, Herradura-, Salobre˜

na- and Adra units. Tubía et al.

(1992), Sanz de Galdeano and L´

opez-Garrido (2003) and Martín-Algarra

et al. (2004) proposed three main units, identied as the Lower-, In-

termediate- and Upper Units. The latter subdivision is followed in this

paper.

The Lower Alpuj´

arride units are composed of Permo-Triassic phyl-

lites and marbles that preserve relics of blueschist-facies metamorphism

dated late-Eocene (Aza˜

n´

on and Crespo-Blanc, 2000; Bessi`

ere et al.,

2022). The Intermediate and Upper units include a portion of Paleozoic

basement composed of high-to medium grade migmatites, gneisses and

graphite schists overlain by Permo-Triassic light-colored schists and

quartzites, phyllites and marbles. In the Western Betics, the Intermediate

unit hosts rare eclogite lenses with Jurassic protolith ages (Tubía and

Gil-Ibarguchi, 1991; S´

anchez-Rodríguez and Gebauer, 2000), whereas

the Upper unit (Los Reales nappe) contains a several kilometer thick

peridotite slab (Ronda peridotites) at the base of the crustal sequence.

In the Montes de M´

alaga and Fuengirola areas, the boundary be-

tween the Upper Alpuj´

arride units and the Mal´

aguide complex has

proven difcult to identify because of similar lithologies and meta-

morphic grades on both sides. In the Montes de M´

alaga, an extra

’Benamocarra unit’ has been dened with transitional characteristics (e.

g. Est´

evez Gonz´

alez and Cham´

on Cobos, 1978; Aldaya et al., 1979;

Elorza and García-Due˜

nas, 1981). Tubía and Navarro-Vil´

a (1984) and

Tubía (1988) placed the contact at the base of a level of brownish schists

containing post-kinematic andalusite and garnet porphyroblasts.

Most structural research in the Alpuj´

arride Complex considers four

main deformation phases (D

-D

), the rst of which corresponds to relics

of a HP/LT foliation (S

) preserved within garnet porphyroblasts and

microlithons of the main S

regional cleavage (e.g. Aza˜

n´

on and Goff´

1997; Booth-Rea et al., 2002). The latter is associated with small-scale

tight-to isoclinal F

folds, except for a possible km-scale structure

interpreted in the Sierra de Lujar (Simancas, 2018). In general, however,

regional-scale folds have been classied as D

structures associated with

crenulation cleavage. They mostly have N- to NW-vergence except in

a zone close to the boundary with the External Zones, where an opposite

vergence is observed as discussed later. The above succession of ductile

structures and fabrics are cut by a series of low-angle detachment faults

and shear bands (D

) with top-to-the-North movement.

is generally attributed to approximately N–S directed Africa-

Eurasia convergence, subduction and crustal shortening in the Eocene

to Oligocene, but its precise kinematics (polarity of subduction) has

remained uncertain. D

has been variably interpreted in terms of

extensional exhumation along a low-angle crustal shear zone (Aza˜

n´

et al., 1997; Balany´

a et al., 1997; Aza˜

n´

on and Crespo-Blanc, 2000;

Alonso-Chaves and Orozco, 2007; Williams and Platt, 2017, 2018;

Simancas, 2018), or transpressional exhumation along a sinistral

NE-trending steep shear zone (Tubía et al., 1992; Rossetti et al., 2005).

structures have been alternatively attributed to top-to-the-N/NE

crustal extension (Orozco et al., 1998, 2004; Platt, 1998) or thrusting

(Cuevas, 1991; Tubía et al., 1992; Simancas and Campos, 1993; Balany´

et al., 1993, 1997, 1998; Aza˜

n´

on et al., 1997; Aza˜

n´

on and Crespo-Blanc,

2000; Simancas, 2018). No agreement exists either on the extensional or

thrust character of late D

faults (Cuevas et al., 1986; Cuevas, 1991;

Simancas and Campos, 1993; Crespo-Blanc et al., 1994; Aza˜

n´

on and

Crespo-Blanc, 2000; Simancas, 2018).

3. Structural relationships in outcrop

Orientation data for crenulation-cleavages collected throughout the

Alpuj´

arride Complex (Figs. 2 and 3) display a bimodal distribution of

steep and shallow dips, which reects the existence of at least two

different age sets exhibiting overprinting relationships in outcrop

(Figs. 4–6). In order to avoid confusion and maintain the traditional

denomination of crenulation cleavage in the Alpujaride Complex as "S

we will further refer to both sets as S

(steep to subvertical dips, ~60-

90◦) and S

(gentle to subhorizontal dips, ~ 0-30◦), but note that we

interpret a different timing for them. Studied lineations are mainly

crenulation lineations, which are usually parallel to intersection and

mineral lineations when found in nearby outcrops.

3.1. Jubrique area

In the Jubrique area (Fig. 2a), S

is strongly overprinted or fully

transposed by a gently west-dipping S

(Fig. 4a, b, 4c, 7 and 8) asso-

ciated with synkinematic andalusite porphyroblasts. S

generally dips

slightly steeper NW to WSW, but locally in opposite (E) direction,

especially in the lower part of the schists and quartzites and in the un-

derlying gneisses. S

reaches its maximum intensity in the middle part

of the schist-quartzite succession. Fold- and crenulation axes have NNE-

SSW to NNW-SSE trends, but a younger set of E-W-trending kink-like

folds was identied in the upper part of the succession.

3.2. Fuengirola area

In the Fuengirola area (south-east of the Sierra Alpujata peridotites;

Fig. 2a) S

is variably overprinted by either an S

or S

(Figs. 4, 5 and

7). South of Sierra Alpujata, S

is well developed and associated with

gently E-dipping shear planes and tight to isoclinal F

folds with E-W

trends. Andalusite porphyroblasts grew synchronous with this foliation

and postdate an earlier sillimanite-bearing schistosity (S

). South of

Benalm´

adena, S

is more widely spaced and associated with relatively

open folds (Fig. 4d).

Between Mijas and Fuengirola (Fig. 2a), two principle sets of S

and

crenulation cleavages were found although their relative timing

could not be determined in this area. The rst strikes NNW-SSE associ-

ated with (Fig. 5a and b) ENE-vergent folds. An additional weaker set of

E-W trending folds is also present, which become increasingly important

in the Torrox and Almu˜

n´

ecar areas further east (see sections 3.3 and

3.4). In the Contraviesa area, overprinting relationships between E-W

and N–S folds (see section 3.6) suggest a later timing of the rst. Be-

tween La Cala de Mijas and Fuengirola and near Benalm´

adena, S

weaker although F

folds with NNW-SSE trends are still abundant.

From there moving westward, the intensity of S

diminishes further

until, near the eastern boundary of the Sierra Alpujata peridotites, it

disappears completely.

3.3. Torrox area

The Torrox area (Fig. 2b) is characterized by a pervasive S

trans-

posing S

(Fig. 4e) in the lower part of the micaschists above the Torrox

orthogneiss body. In higher levels, S

is weaker or absent but F

folds

are locally still prominent. A large number of orientation data for folds

and lineations collected by Alonso-Chaves and Orozco (2012) com-

plemented with our own data (Fig. 7) indicate a broadly bimodal trend

distribution with a strong E-W maximum and a more dispersed

NNW-SSE maximum. Note that fold axes and lineations measured in the

Fuengirola area (Fig. 7), although less numerous, exhibit a similar

bimodal distribution (section 3.2).

3.4. Almu˜

n´

ecar area

In the Almu˜

n´

ecar area (Fig. 2b), S

is generally less intense as in the

Torrox area but still locally transposes S

folds (Fig. 4f) have ENE-

WSW to E-W trends and inconsistent vergences. An L

lineation is visible

in outcrop parallel to fold axes. In the southernmost part of the area,

between La Herradura and Salobre˜

na, a steeply dipping widely spaced

crenulation cleavage (S

) with ENE-WSW strike and northward ver-

gence was observed to overprint S

(Fig. 5c and d).

3.5. Toc´

on de Qu´

entar – Sierra de Baza area

In the Toc´

on de Qu´

entar area (Fig. 3a), micaschist outcrops are

A. Ruiz-Fuentes and D.G.A.M. Aerden

Journal of Structural Geology 168 (2023) 104823

Fig. 2. Geological maps of the Jubrique-Fuengirola (A) and Torrox-Almu˜

n´

ecar (B) areas showing the location of cross sections, outcrop and microstructural images

(Figure numbers in small boxes), average microstructural trends indicated with red, orange and green bars corresponding to the three trend ranges dened in Fig. 11,

and tectonic transport directions (thin arrows) deduced from inclusion-trail asymmetries. (For interpretation of the references to color in this gure legend, the reader

is referred to the Web version of this article.)

A. Ruiz-Fuentes and D.G.A.M. Aerden

Journal of Structural Geology 168 (2023) 104823

scarce and correspond to small weathered klippe located on top of

carbonate rocks by means of low angle faults probably related to the D

low-angle faults and shear bands in the Contraviesa area. The main

foliation is overprinted by a spaced S

crenulation cleavage associated

with ENE-WSW- to NE-SW trending folds (Fig. 5e and f, 7). Interestingly,

the vergence of these folds is southward (Fig. 5e), opposite to that

observed in the Contraviesa area. Orientation data for pre-D

folds and

lineations suggest two sets of structures with NNW-SSE versus NW-SE to

WNW-ESE trends.

3.6. Contraviesa area

In the Contraviesa area (Fig. 3b), S

dips moderately to steeply south

(Fig. 7) and is overprinted with variable intensity by WSW-ENE striking

Fig. 3. Geological maps of the Toc´

on de Qu´

entar (A) and Contraviesa (B) areas showing the location of cross sections, outcrop and microstructural images

(Figure numbers in small boxes), average microstructural trends indicated with red, orange and green bars corresponding to the three trend ranges dened in Fig. 11,

and the directions of tectonic transport (thin arrows) deduced from inclusion-trail asymmetries. Map ‘A’ is partially based on Sanz de Galdeano et al. (1995). (For

interpretation of the references to color in this gure legend, the reader is referred to the Web version of this article.)

A. Ruiz-Fuentes and D.G.A.M. Aerden

Journal of Structural Geology 168 (2023) 104823

or S

(Fig. 5g, h, 8b, 8c). As previously described by Cuevas (1991),

is particularly well developed in the Upper Alpuj´

arride unit (Adra

nappe) cropping out in the south of the Contraviesa area. Within the

Intermediate Alpuj´

arride unit (Murtas nappe) further north, S

be-

comes scarcer as D

structures make their appearance (Fig. 8). The

latter correspond to south-verging tight to open folds with subhorizontal

axial planes and top-to-the north shear bands and faults which postdate

structures (Fig. 4g, h and 5g). F

folds and L

lineation have variable

plunges and plunge directions, ranging from SE to SW, and dene a

broad N–S maximum (Fig. 7). Plunge directions of F

and F

folds

show an even larger spread from E to S to W with a weak E-W maximum.

In several outcrops an E-W to NE-SW striking S

was observed

deforming an older steeply WSW dipping foliation, and resulting in very

steeply SW plunging fold- and crenulation axes. This is consistent with

structural data of Aza˜

n´

on et al. (1997) from the eastern Sierra de la

Contraviesa, which also show early NNE-SSW trending lineation and

folds (their ’L

’) overprinted by ENE-WSW trending folds (their ’F

’

Thus, the weakly bimodal distribution of D

and D

linear structures in

the Contraviesa area probably reects the superposition of ENE-WSW

structures over older N–S ones. Note that equivalent structural data

for the Toc´

on, Torrox and Fuengirola areas all exhibit similar bimodal

trends (Fig. 7).

4. Microstructural analysis

4.1. Microstructural approach

A total of 140 samples of medium-to high-grade metapelites con-

taining garnet, staurolite, plagioclase and/or andalusite porphyroblasts

(Figs. 9 and 10) were studied in thin section to analyze mineral re-

lationships and to select appropriate samples for further microstructural

analysis in additional differently oriented thin sections and X-ray

computed micro-tomographies (XCT) (see XCT method on Supplemen-

tary Material SM1). SM2 (Supplementary material) lists the 60 selected

samples, their locations and corresponding nappe units. Fig. 11a shows

the general relationships between the principle index minerals in these

rocks and the traditionally distinguished deformation phases. Silli-

manite is a good marker of the high-temperature stage in high-grade

rocks linked to the D

event. Andalusite mainly grew during D

(Cue-

vas, 1989; Aza˜

n´

on et al., 1998; Williams and Platt 2017), although we

actually recognized three different types of andalusite porphyroblasts:

(i) small equidimensional crystals with well-developed straight to

sigmoidal inclusion trails indicating syn-D

growth (Fig. 10c, d and e),

(ii) up to several cm long post-kinematic crystals overgrowing all matrix

fabrics (Fig. 10f), and (iii) large crystals usually devoid of inclusion trails

Fig. 4. Outcrop images of S

. A) Intensely

developed S

in quartzites situated near the

middle of the Jubrique schist-quartzite for-

mation. B) Widely spaced shear bands in the

lower levels of the same formation. C) F

folds in gneisses of the Jubrique Fm. D)

Folds associated with S

in the

Benalm´

adena area. E) S

near Torrox

transposing S

in a pelitic layer above a less

deformed psammitic layer showing F

folds. F) Interference of F

and F

folds near

La Herradura. G) Coeval D

folds and shear

bands in the Contraviesa area. H) Sub-

horizontal shear bands south of Albond´

genetically related to S

A. Ruiz-Fuentes and D.G.A.M. Aerden

Journal of Structural Geology 168 (2023) 104823

which appear broken, stretched (Fig. 10g) and partially replaced by

ne-grained quartz-mica aggregates. The third type possibly corre-

sponds to the pre-Alpine andalusite crystals reported by S´

anchez-Navas

et al. (2012) in the vicinity of the Torrox orthogneiss.

Whilst garnet porphyroblasts in our samples of Permo-Triassic

metasediments can only be Alpine, those in Paleozoic dark schists could,

in principle, also be Variscan. Nevertheless, an Alpine origin in both rock

types is considered likely because (i) evidence for Variscan garnet

growth has previously only been found in high-grade gneisses not

studied herein, (ii) inclusion trails of garnets in samples from cover and

Fig. 5. Outcrop images of S

. A) S

intensely developed and becomes the main

foliation in outcrop south of Mijas. Fig. 10a

shows a microscope image of the S

cren-

ulations. B) Intensely folded gneisses south

of Mijas. C) S

overprinting D

folds east

of Almu˜

n´

ecar. Fig. 10b shows a microscopic

image of the same outcrop. D) Fold inter-

ference in amphibolites of the Herradura

nappe west of Almu˜

n´

ecar with S

over-

printing S

, in turn overprinting an F

fold.

Fig. 10h shows a microscope image of the

same outcrop. E) South-vergent D

folds

north of Toc´

on de Qu´

entar. F) N–S L

over-

printed by E-W F

folds north of Toc´

on de

Qu´

entar. G) D

folds cut by D

shear bands

in the Contraviesa area. H) S

partially

transposing S

in the Contraviesa area.

Fig. 6. Equal-area and lower-hemisphere projections for: total porphyroblast FIAs measured in all areas, all fold axes and lineations shown separately for different

areas in Fig. 7, and a similar compilation of poles to all crenulation cleavages shown in Fig. 7. All equal-area projections were made with the program ’Stereonet’ by

Allmendinger et al. (2013) and use Kamb contours with contour interval and signicance level set to 2 or 3

(see gure).

A. Ruiz-Fuentes and D.G.A.M. Aerden

Journal of Structural Geology 168 (2023) 104823

Fig. 7. Equal-area and lower-hemisphere projections for: porphyroblast FIAs measured in different areas, F

(black dots) and F

(red dots) fold axes and

associated lineations measured in outcrop, and poles to the main foliation (S

; black dots) and superposed crenulation cleavages (S

; red dots). All equal-area

projections were made with the program ’Stereonet’ by Allmendinger et al. (2013) and use Kamb contours with both contour interval and signicance level set

to 2

A. Ruiz-Fuentes and D.G.A.M. Aerden

Journal of Structural Geology 168 (2023) 104823

basement rocks have similar orientations as shown further, and (iii)

Sm–Nd ages of garnets in ve Paleozoic schist samples from the

Alpuj´

arride- and Sebtide complexes are all Alpine (Farrell, 2019; Aerden

et al., 2022).

The studied porphyroblasts host straight to weakly sigmoidal to

spiral-shaped inclusion trails composed of quartz, graphite and/or

opaque minerals (Figs. 9 and 10), although inclusion trails-free por-

phyroblasts are also common. Samples were initially studied in hori-

zontal thin sections (regardless of the orientation of the matrix foliation

and lineation), because of the advantage that the strike of relatively

straight inclusion trails can be directly measured on such sections.

Samples were classied in three groups. A rst group of 36 samples

contains garnet porphyroblasts with well-developed sigmoidal or spiral

inclusions whose FIA can be readily measured in XCT scans. Some of

these samples also contain planar inclusion trails whose orientations

were also measured. A second group of 15 samples contains asymmet-

rically curved inclusion trails, but either less well developed in garnet

porphyroblasts or well developed but hosted by other porphyroblastic

minerals (andalusite, plagioclase, staurolite), which are less apt for XCT

analysis because of a lower X-ray attenuation contrast with their mineral

inclusions and the matrix. These samples were studied using the radial

thin-sectioning technique of Hayward (1990) and Bell et al. (1995) by

which the average orientation of porphyroblast FIAs in a sample can be

determined. As we cut six vertical thin sections for each sample, our

average FIAs are constrained to 30◦trend ranges. A third group of 9

samples only contains porphyroblasts with relatively straight inclusion

trails, whose strikes were measured in the initially cut horizontal thin

sections. Microstructural data obtained from XCT scans and thin sections

are presented in contoured equal-area projections made with ’Stereonet’

(Allmendinger et al., 2013) and in moving-average rose diagrams,

respectively, for each sample. The rose diagrams were made with the

program ‘MARD’ (Munro and Blenkinsop, 2012). Due to space limita-

tions, these diagrams are provided as supplementary data together with

a table summarizing the mean microstructural orientation in each

sample (Supplementary Material SM3, SM4 and SM5).

4.2. Relative timing criteria and microstructural sequence

Mean microstructural trends dened by FIAs and/or inclusion-trail

strikes are plotted in rose diagrams for all porphyroblastic minerals

collectively (Fig. 11b) and also separately for garnet versus plagioclase,

andalusite and staurolite porphyroblasts (Fig. 11c). Three sets of mi-

crostructures are visible in these plots that dene NNW-SSE (green),

ENE-WSW (orange) and WNW-ESE (red) orientation maxima. Note that,

whereas all three sets are present in garnet porphyroblasts, andalusite,

staurolite and plagioclase porphyroblasts almost exclusively contain the

NNW-SSE and ENE-WSW sets.

Relative timing criteria for microstructures with different trends

include: (1) porphyroblasts with differently oriented FIAs in the core

versus the rim (Fig. 12a and b), (2) porphyroblasts containing inclusion

trails with different strikes in the core versus rim (Fig. 12c), (3) weakly

crenulated inclusion trails whose axial planes can be taken as the di-

rection of an incipient foliation (Fig. 12d), (4) inclusion trails hosted in

garnet versus staurolite and andalusite, since the latter formed later (e.g.

Balany´

a et al., 1997; Aza˜

n´

on et al., 1997; Williams and Platt, 2017), (5)

crenulation lineations in the matrix versus FIAs.

Fig. 11e summarizes timing relationships found in our samples

following the above criteria. Mean microstructural trends are repre-

sented with red, orange or green symbols depending on their association

with the WNW-ESE, ENE-WSW or NNW-SSE modal maxima in the rose

diagram of Fig. 11b. In the case of a mean microstructural trend falling

within the overlap zones between two trend groups its assignment to one

of these groups was based on relative timing criteria with other micro-

structural trends in the sample or in nearby samples. The trends of

matrix crenulations and fold axes measured in samples or their outcrops

are also represented (star and triangle symbols).

Mutual relative-timing criteria between inclusion trails in porphyr-

oblasts (ve samples) suggest that WNW-ESE FIAs (red) formed rst,

followed by ENE-WSW FIAs (orange) and nally NNW-SSE ones (green).

This succession is supported by the fact that matrix lineations and folds

mainly have ’green’ and ’orange’ trends and that ’red’ FIAs almost

Fig. 8. Cross sections of the Jubrique (A-A

′) and Contraviesa (B–B′and C–C′) areas. White numbers in black squares correspond to Figure numbers.

A. Ruiz-Fuentes and D.G.A.M. Aerden

Journal of Structural Geology 168 (2023) 104823

exclusively occur within garnet porphyroblasts (Fig. 11c), whereas the

two younger FIA sets are found equally in garnet, staurolite, andalusite

and plagioclase porphyroblasts. One sample (63.8.1) contains a ’red’

FIA hosted by staurolite, which probably grew synchronous with garnets

in other samples.

Sample 68.4.1, however, paradoxically contains a ’red’ FIA included

by late (syn-D

) andalusite porphyroblasts post-dating garnet growth.

Two possible explanations are (1) that these andalusites grew in a zone

where ’red’ structural trends were preserved due to the partitioning of

later deformations around it, or (2) grew in a pre-existing plunging fold

which can create FIAs oblique to the shortening direction (see Fig. 5e of

Aerden et al., 2022). Another contradictory relationship was found in

sample 66.11.1, where garnets with ’green’ FIAs are surrounded by L

lineation and F

folds oriented within the ’red’ trend range (133/05).

The proximity of this direction to the overlap zone with the ’green’ trend

range and the fact that neighboring sample 66.10.1 hosts similar matrix

structures but trending slightly more NNW-SSE within the ’green’ trend

range suggests that 66.11.1 is an outlier and in fact belongs to the

’green’ trend group. All other relative timing criteria between FIAs

versus L

and F

folds are consistent with the sequence of ’red’, ’orange’

and ’green’ microfabrics.

Relative timing relationships between FIAs and L

lineations and F

folds imply a repetition of ’orange’ fabrics after ’green’ ones. This fol-

lows from nine samples containing E-W to NE-SW trending L

sur-

rounding inclusion trails with ’green’ FIAs or strikes (Fig. 11e).

Furthermore, syn-D

andalusite porphyroblasts preserve both ’green’

and ’orange’ FIAs that necessarily post-date all garnet FIAs and S

Indeed, both S

and S

matrix crenulations deform sillimanite bers

associated with S

(Figs. 9h, 10a and 10b, 10c and 11a; Cuevas 1989;

Aza˜

n´

on et al., 1998; Williams and Platt 2017)

4.3. Regional distribution of microstructural trends

FIAs, lineations and fold axes in our samples and their outcrops are

shown in the maps of Figs. 2 and 3 with red, orange or green trend bars.

The curvature sense or asymmetry of sigmoidal to spiral-shaped inclu-

sion trails is symbolized in these maps with small arrows drawn normal

to FIA trend bars. These arrows are opposite to the direction of tectonic

transport that would be traditionally deduced from the asymmetry of

inclusion-trails, assuming a ’rotational’ origin, but point in the correct

direction according to the non-rotational interpretation (Bell and

Johnson, 1989) favored in this paper (see section 5.1). For ease of dis-

cussion, inclusion-trail asymmetries are therefore specied in terms of

tectonic transport directions predicted by the ’non-rotational’ model.

Fig. 9. Microstructural images (taken under

plane polarized light except D which is an

XCT image; half arrows indicate the upward

direction together with the normal direction

indicating the horizontal, geographic direc-

tion with numbers). A) Garnets with

sigmoidal inclusion trails. The S

foliation

and relics of S

are visible in the matrix. B)

Garnet with sigmoidal inclusion trails with

internal truncations. C) Syn-S

garnets of the

Benamocarra unit in microlithons of an

incipient S

foliation. D) XCT image of gar-

nets hosting spiral shaped inclusion trails.

Same sample as in B for comparison. E)

Staurolite porphyroblasts whose straight in-

clusion trails are wrapped by S

(plain view,

north is indicated by the arrow). F) Stauro-

lite porphyroblast with weakly sigmoidal

inclusion trails. G) Plagioclase with straight

inclusion trails including tiny garnet por-

phyroblasts. H) S

folding sillimanite asso-

ciated with S

in the Fuengirola area. Note

local shear-band character of S

A. Ruiz-Fuentes and D.G.A.M. Aerden

Journal of Structural Geology 168 (2023) 104823

That is, a clockwise sigmoid or spiral corresponds to top-to-the right

tectonic transport.

Different FIA sets are heterogeneously distributed in the Alpuj´

arride

complex. In the Western Betics (Fig. 2a) only ’green’ and ’orange’ FIAs

have been found in 14 samples with the exception of sample 70.6.1,

which contains both ’orange’ and ’red’ FIAs. Particularly noteworthy is

the consistency of FIA orientations in the Fuengirola and Jubrique areas

despite very different trends of matrix lineations and fold axes in both

regions (E-W to SE-NW versus N–S, respectively). In the Almu˜

n´

ecar-

Torrox area (Fig. 2b), signicant differences can be noticed between the

data from samples collected west and east of Nerja. West of this town,

garnets mainly preserve ’red’ and ’green’ FIAs, whereas east of it all

three FIAs are roughly equally represented and this is further maintained

to the east in the Toc´

on de Qu´

entar and Contraviesa areas (Fig. 3a and

b).

5. Interpretation and discussion

5.1. Formation mechanism of inclusion trails

Consistent inclusion-trail orientations in metamorphic belts (see

Introduction) have been mostly explained in terms of a model

envisaging porphyroblast nucleation and growth within actively devel-

oping microlithon domains without much porphyroblast rotation (Bell,

1985; Fay et al., 2008; Bell and Fay 2016). According to this model,

sigmoidal and spiral-shaped inclusion trails form by overgrowth of one

or multiple crenulations. Furthermore, preferred subvertical and sub-

horizontal orientations of inclusion trails documented in different oro-

gens (Bell et al., 1992; Hayward, 1992; Johnson, 1992; Aerden, 1994,

1995, 1998, 2004; Mares, 1998; Sayab, 2005; Shah et al., 2011; Bell and

Sapkota, 2012; Aerden et al., 2013; Aerden and Ruiz-Fuentes, 2020) are

inferred to reect alternations of crustal shortening and transient

gravitational collapse stages. FIAs resulting from this process should

have subhorizontal plunges and trends normal to the crustal shortening

direction.

Several general features of our microstructural data, and similar ones

described by Aerden et al. (2022, their Fig. 6) support the above sum-

marized ’non-rotational’ model in the Betic-Rif orogen and its

predictions:

•Our FIAs have mainly sub-horizontal to gentle plunges regardless of

their trend or timing (Fig. 6). If they had formed by shearing-induced

porphyroblast rotation, then it is difcult to explain why older FIAs

Fig. 10. Microstructural images (taken

under plane polarized (A, B, C, D, E, F, H) or

cross polarized light (G); half arrows indi-

cate the upward direction together with the

normal direction indicating the horizontal,

geographic direction with numbers). A)

Detail of outcrop image of Fig. 5a. Silli-

manite is folded by S

. B) Detail of outcrop

image of Fig. 5c. S

crenulates a sillimanite-

bearing S

. C) Syn-D

andalusite porphyr-

oblasts include sillimanite-bearing S

. D)

Syn-D

andalusite porphyroblasts of the

Jubrique area. E) S

partially transposes S

and is associated with andalusite growth.

Jubrique area. F) Post-kinematic andalusite

porphyroblasts of the Toc´

on de Qu´

entar

area. G) Stretched andalusite porphyroblast

adjacent to the Torrox gneiss which might be

Variscan. H) Incipient S

affects S

and S

in the outcrop of Fig. 5d.

A. Ruiz-Fuentes and D.G.A.M. Aerden

Journal of Structural Geology 168 (2023) 104823

Fig. 11. A) Summary of relationships between mineral growth and deformation phases. B) Rose diagram compiling all mean FIA trends and inclusion-trail strikes

listed in SM5. Three microstructural trends (MT-1, MT-2 and MT-3) are indicated associated with modal maxima at N070, N110, N170 and trend ranges N140–N090,

N090–N030, N140–N210. Structures falling within these trend ranges are colored red, orange and green, respectively, in E and F. C) Same data separated for garnets

versus andalusite, staurolite and plagioclase. D) Summary of inclusion trail pitches measured in sections oriented normal to FIA in samples containing a single or very

dominant FIA set. Data from individual samples can be found in SM6. E) Temporal relationships between (micro)structures with different trends in individual

samples based on inclusion-trail geometry alone (core-rim relationships, truncational relationships, crenulated inclusion trails), inclusion trails versus crenulation-

and fold axes in the matrix (S

and S

dominated), and overprinting relationships between fold axes and lineations mutually. Arrows point from older to younger

elements and are summarized by four circular diagrams, where numbers indicate how many times each temporal relationship is found. F) Rose diagrams for FIAs,

fold axes and lineations plotted in Fig. 7 and color-coded according to the three trend ranges dened in B. (For interpretation of the references to color in this gure

legend, the reader is referred to the Web version of this article.)

A. Ruiz-Fuentes and D.G.A.M. Aerden

Journal of Structural Geology 168 (2023) 104823

were not reoriented and steepened during the development of

younger ones.

•Dip angles of internal foliations measured in porphyroblast cores

show a bimodal distribution with strong preferences for steep or at

lying (Figs. 11d and 13 and SM6). Axial planes and truncations

associated with more complex inclusion-trail patterns exhibit similar

at-steep patterns (Fig. 13).

•FIA trends have consistent orientations in samples and areas where

matrix structures and fabrics have totally different orientations

(Fig. 2a).

•The shear sense indicated by asymmetric pressure shadows of por-

phyroblasts often conicts with the sense of inclusion-trail curvature

if porphyroblast rotation is assumed (Fig. 14).

5.2. Kinematic signicance of ’S

’ inclusion trails

Williams and Platt (2017) already noted that garnet porphyroblasts

occasionally host crenulated inclusion trails, which imply a polyphase

character of ’D

’. The three sets of inclusion trails distinguished in this

paper suggest a superposition of three regional-scale kinematic frames

further referred to as D

, D

and D

(Fig. 15). Aerden et al. (2022)

recently recognized the same three FIA sets based on microstructural

data that included 150 individual FIAs measured with XCT in 12 samples

of the Sebtide Complex (the African equivalent of the Alpuj´

arrides),

another 150 individual FIAs in ve samples of the Nevado-Fil´

abride

Complex complemented with 87 average FIAs for the Nevado-Fil´

abride

Complex, but only 47 individual FIAs from seven samples of the

Alpuj´

arride Complex. Our expansion of the data for the latter to 647

individual FIAs plus 25 average FIAs from 60 samples conrms the

orogen-wide character of the three FIA directions and supports a rela-

tionship with the plate motion history of the Mediterranean Alpine belt

proposed by Aerden et al. (2022).

These authors showed, based on Sm–Nd garnet ages and published

plate-motion reconstructions (Rosenbaum et al., 2002; Vissers and

Meijer, 2012; DeMets et al., 2015), that the ’red’ FIA set (D

) probably

formed in the latest-Eocene to early Oligocene perpendicular to NNE

Fig. 12. Examples of temporal relationships between microstructural trends indicated by different FIAs (A and B; XCT images; half arrows indicate the upward

direction together with the normal direction indicating the horizontal, geographic direction with numbers), different inclusion-trail strikes (C; XCT image; plain view,

north is indicated by the arrow) in core versus rims of garnets, or by weakly crenulated inclusion trails (D; microphotograph, plane polarized light; plain view, north

is indicated by the arrow) in plagioclase. The microstructural trends are color-coded according to the three trend ranges dened in Figure 11b.

A. Ruiz-Fuentes and D.G.A.M. Aerden

Journal of Structural Geology 168 (2023) 104823

directed motion of Africa relative to Eurasia. Subsequent ’orange’ FIAs

) dated between 27Ma and 22Ma in the Alpuj´

arride-Sebtides would

have developed during or after an anticlockwise rotation of the

plate-motion vector to NW. The third set of ’green’ FIAs (D

) was

attributed to westward extrusion of the Albor´

an Domain in the early

Miocene, approximately normal to Africa-Iberia convergence.

Aerden et al. (2022) also speculated about the polarity of subduction

as suggested by the curvature sense or asymmetry of inclusion trails (see

below). However, their asymmetry data were limited to 14 samples of

the Sebtide Complex, six from the NFC and only four from the

Alpuj´

arride Complex. Our extension of this data to 42 Alpuj´

arride

samples now allows statistically more signicant conclusions. Fig. 16

shows the asymmetries recorded in samples from different areas of the

Alpuj´

arride and Sebtide complexes. Of 25 samples hosting D

FIAs, 13

exhibit top-SSW asymmetries, 9 top-NNE, and 3 contain porphyroblasts

showing opposite asymmetries (Figs. 2 and 3). The top-NNE asymme-

tries mainly correspond to the Benamocarra unit as mapped by Williams

and Platt (2018) and the Intermediate Alpuj´

arride unit (Herradura

nappe). In the Upper Alpuj´

arride units (Adra and Salobre˜

na nappes) and

the Sebtides, D

asymmetries very consistently indicate top-SSW

transport thus favoring a northward dipping subduction zone (Figs. 16

and 17). The asymmetries of D

(orange) FIAs also exhibit a strong

predominance (Fig. 16; 19 to 7) of top-S to -SE, which is consistent with

generally accepted NW-dipping subduction of the African plate below

the Albor´

an domain in the late Oligocene to early-Miocene (Fig. 17).

Interestingly, the asymmetry of D

FIAs recorded in two Benamocarra

samples are again opposite (top-NW) to that in the underlying Upper

Alpuj´

arride unit. The third set of inclusion-trails (D

) has a top-west

asymmetry in 11 samples, top-east asymmetry in 11 samples and

opposite asymmetries in 3 samples. This may reect distributed coaxial

shortening in the Alpuj´

arride complex driven by westward migration of

the Albor´

an Domain in the Miocene without a dominant vergence.

The predominance of gentle FIA plunges and the bimodal steep-at

orientations of inclusion trails detected in our samples (Figs. 6, 11 and

13 and SM6) implies an unspecied number of alternations between

crustal shortening and transient gravitational instability (collapse)

during D

, D

and D

1C.

Presumably these reect uctuations in a

critical balance between tectonic stresses and gravity controlled, in turn,

by variations in the rates of erosion, uplift, and plate convergence, the

thermal and rheological evolution of the orogen and changes in

boundary conditions. Intermittent collapse phases during D

must have

been relatively weak compared to shortening phases as they are not

reected in cyclic prograde-retrograde metamorphism. Collapse events

may correspond to gravitational spreading within thrust nappes in an

overall contractional setting, alternating with periods of distributed

shortening. In any event, these alternations were sufcient to control

episodic growth of porphyroblasts with each growth pulse being trig-

gered by a newly developing subvertical or subhorizontal crenulations

(cf. Bell and Hayward, 1991; Sanislav and Bell, 2011; Aerden and

Ruiz-Fuentes, 2020). This mechanism also explains the preponderance

of subhorizontal or gently plunging FIAs.

Two samples from the Almu˜

n´

ecar area (66.9.1 and 62.5.2), however,

contain very steeply plunging FIAs (Fig. 7 and SM3) corresponding to

the intersection of two steeply dipping foliations with different strikes

Fig. 13. Line tracings of inclusion trails drawn on high-resolution microphotographs of vertical thin sections oriented approximately normal to porphyroblast FIAs.

Half arrows indicate the upward direction together with the normal direction indicating the horizontal, geographic direction with numbers. A) Inclusion trails in

sample 64.4.1 exhibit subhorizontal (blue lines) and subvertical (red lines) axial planes, consistent with the garnets having nucleated at least twice during different

crenulation forming events related to crustal shortening and collapse. B) Complex sigmoidal and spiral shaped inclusion trails in sample 66.10.1 associated with

internal truncations (blue and red lines). These elements show similar preferred orientations as the axial planes in A, consistent with episodic growth pulses of

individual porphyroblasts controlled by contraction-collapse cycles (e.g. Bell and Hayward, 1991). (For interpretation of the references to color in this gure legend,

the reader is referred to the Web version of this article.)

Fig. 14. Andalusite (A; sample 70.3.1) and

staurolite (B; sample 64.6.1) porphyroblasts

containing weakly sigmoidal inclusion-trails

that have the same asymmetry as shear

bands and strain shadows (microphoto-

graphs taken under plane polarized light;

half arrows indicate the upward direction

together with the normal direction indi-

cating the horizontal, geographic direction

with numbers). Note that this is consistent

with a ’non-rotational’ interpretation of the

inclusion trails as overgrown crenulations.

A. Ruiz-Fuentes and D.G.A.M. Aerden

Journal of Structural Geology 168 (2023) 104823

not separated by a at lying crenulation cleavage. In the same area,

straight inclusion trails which dene internal foliations with steep dips

are particularly abundant (samples 66.6.1, 60.2.2, 60.3.1, 66.4.1)

possibly reecting more limited role of gravitational collapse in com-

parison with other areas.

5.3. Kinematics of S

Even in individual thin sections, the character of the main cleavage

varies from a differentiated S

crenulation cleavage deforming S

in the

shortening eld of D

to a simple schistosity that can be followed into

garnet porphyroblasts and represents an S

that was deformed and

reactivated in the extensional eld of D

. This composite character of the

’main foliation’ and the fact that the process of foliation reactivation

involves antithetic shearing (cf. Bell et al., 1986; Aerden, 1994) may

explain partially contradictory shear-sense indicators associated with S

Nevertheless, most workers have concluded top-to-the-East to -NNE

tectonic transport during D

with a component of coaxial shortening

normal to S

. The development of S

was accompanied by strong

decompression related to opening of the western Mediterranean

back-arc basin leading to sillimanite growth in the higher-grade rocks.

Such an extensional origin of S

suggests it formed in a subhorizontal

position although we have no rm data conrming this. This event

(Fig. 17) is related with high-Temperature metamorphism dated at

approximately 20 Ma (e.g. Platt and Whitehouse, 1998; Platt et al.,

2005; Bessi`

ere et al., 2022), which is also the age of the oldest uncon-

formable marine deposits on top of the Internal Domain (Vi˜

nuela Group,

e.g. Aguado et al., 1990; Alonso-Chaves and Rodríguez-Vidal, 1998) and

in the Albor´

an basin (Comas et al., 1992; Rodríguez-Fern´

andez et al.,

1999).

Fig. 15. A) P-T-t paths of different tectonic units of the Alpuj´

arride complex. The conditions of the rocks studied in this work correspond to garnet-sillimanite schists

of the Upper Alpuj´

arride units. The paths of phyllites and kinzigites of the Upper Alpuj´

arride units, and of the Intermediate Alpuj´

arride units are also shown for

comparison. Circles, squares and stars show the approximate location of deformation phases along the path as interpreted by the original authors. (1) Eclogites of the

Oj´

en nappe (Tubía and Gil-Ibarguchi, 1991); (2) sillimanite schists of the Los Reales nappe near Fuengirola (Tubía, 1994); (3) kinzigites of the Los Reales nappe near

Fuengirola (Tubía, 1994); (4) light colored schists of the Tejeda unit, equivalent to Intermediate Alpuj´

arride units or Herradura nappe (Aza˜

n´

on and Alonso-Chaves,

1996); (5) garnet schists of the Adra nappe (Aza˜

n´

on et al., 1997); (6) phyllites of the Jubrique unit (Balany´

a et al., 1997); (7) sillimanite schists of the Jubrique unit

(Balany´

a et al., 1997); (8) kinzigites of the Jubrique unit (Balany´

a et al., 1997); (9) phyllites of the Salobre˜

na nappe (Aza˜

n´

on et al., 1998); (10) sillimanite schists of

the Salobre˜

na nappe (Aza˜

n´

on et al., 1998); (11) Bentomiz unit, Upper Alpuj´

arride nappe from the Torrox area (Alonso-Chaves and Orozco, 2007). B) Schematic P-T-t

trajectory for the higher grade (sillimanite bearing) rocks studied in this work based on A. The timing of deformation events distinguished in this work are tentatively

indicated using the same colors (red, orange, green) as attributed in Fig. 11 to differently oriented structures. Gravitational collapse stages are colored in pink. (For

interpretation of the references to color in this gure legend, the reader is referred to the Web version of this article.)

Fig. 16. Table summarizing the asymmetries of inclusion trails from garnet, staurolite and plagioclase found in each area studied (Upper and Intermediate

Alpuj´

arrides and Benamocarra unit) and the Rif (in northern Morocco).

A. Ruiz-Fuentes and D.G.A.M. Aerden

Journal of Structural Geology 168 (2023) 104823

Fig. 17. Paleogeographic maps showing the evolution of the Albor´

an Domain in each phase distinguished in this study.

A. Ruiz-Fuentes and D.G.A.M. Aerden

Journal of Structural Geology 168 (2023) 104823

5.4. Kinematics of D

The tectonic signicance of major N- to NW verging F

folds in the

Alpuj´

arride Complex and associated S

crenulation cleavage has been

debated since long. In the Central Betics, some authors interpreted these

structures as late-metamorphic N-to NE-directed folds and thrusts

following D

extension (Cuevas, 1991; Simancas and Campos, 1993;

Aza˜

n´

on et al., 1997; Balany´

a et al., 1997; Rossetti et al., 2005; Simancas

2018). Others envisaged their development during continued crustal

extension but with a changed top-N shear sense (Orozco et al., 1998,

2004, 2017; Williams and Platt, 2018). In the Fuengirola area, Tubía

et al. (1993) and Tubía (1994) interpreted D

as a phase of ESE-WNW

sinistral transtension changing to ENE-WSW sinistral transpression.

Orozco et al. (1998) and Rossetti et al. (2005) locally recognized two

crenulation cleavages (corresponding to our S

and S

) and Sanz de

Galdeano (1989) described a superposition of two E-W and N–S folding

directions in Triassic marbles of the Sierra de Tejeda (North of the

Torrox area; Fig. 2b) both deforming an original ’main schistosity’.

Our own observations regarding D

can be summarized as follows

(Figs. 15 and 17). In the Western and Central Alpuj´

arrides (Jubrique,

Fuengirola and Almu˜

n´

ecar areas), a penetrative S

is associated with

synkinematic andalusite or sillimanite and is overprinted by late- to

post-metamorphic S

(Fig. 5c, d, 10a, 10b). In the Contraviesa area, an

opposite situation is found with steeply south-dipping S

locally over-

printed by a gently dipping S

and post-metamorphic shear bands. The

latter corresponding to the ’D

’ structures of previous workers. In the

intermediate Torrox-Almu˜

n´

ecar area, S

is still the dominant crenula-

tion cleavage but less intense as in the Jubrique schists and locally

overprinted by D

. Thus, a west-ward strain gradient associated with

is suggested. D

has only been found pervasively developed in the

Fuengirola and Contraviesa areas. In the latter, zones of intense S

and

tight folding of S

alternate with zones with a simple steeply south

dipping S

. Cuevas (1991) interpreted the intense D

folded zones as

ductile thrusts supported by quartz c-axes fabrics indicating top-NE

shearing.

These heterogeneous relationships suggest that, following several

alternations of crustal shortening and weak gravitational collapse stages

that generated the three FIA sets preserved in garnets (D

), a more

dramatic collapse event occurred (D

and D

) related to opening of the

western Mediterranean basin that was still followed by renewed crustal

shortening responsible for S

. The late subhorizontal shear bands and

detachments (D

of previous workers) may reect a nal phase of

thrusting and gravitational spreading following uplift caused by D

(cf.

Platt et al., 1983). Interestingly, several authors already interpreted

-D

in terms of alternating shortening and extension (Tubía et al.,

1993; Aza˜

n´

on et al., 1997, 1998; Balany´

a et al., 1997). Such a scenario

appears conrmed and rened by our work.

The fact that ’green’ and ’orange’ FIAs of andalusite porphyroblasts

must have formed after ’green’ FIAs of garnets, and that ’green FIAs’ in

both minerals are surrounded by ’orange’ D

fabrics in several samples

was stated in section 4.2 as implying development of ’orange’ FIAs

before and after ’green’ FIAs. We suggest that this reects an alternation

of two suborthogonal shortening directions related to NW motion of

Africa and simultaneous westward motion of the Albor´

an Domain.

Regional fold trends also dene two sets with E-W and N–S to NW-SE

trends in all investigated areas (Fig. 7). In the westernmost part of the

Gibraltar arc the N–S set is dominant (Fig. 1), whereas towards the east

E-W to NE-SW trends become more important (Balany´

a and

García-Due˜

nas, 1987; Balany´

a, 1991; Balany´

a et al., 2007).

5.5. Back-thrusting development

In the Toc´

on de Qu´

entar and Sierra de Baza areas, E-W trending D

folds are S-vergent, opposite to similar structures in the Almu˜

n´

ecar and

Contraviesa areas. South-vergent structures have been previously also

recognized in Sierra de Baza (Comas et al., 1979; Delgado Salazar et al.,

1980), Sierra de las Estancias (Akkerman et al., 1980), Sierra de Alma-

gro (García-Tortosa et al., 2002; Booth-Rea et al., 2005), Sierra de

Carrascoy (Sanz de Galdeano et al., 1997) and in the Orihuela Moun-

tains (Martín-Rojas et al., 2007) (see locations in Fig. 1). S-vergent D

folds are concentrated near the Internal-External Zones Boundary

(IEZB), which represents the suture of the collision between the Albor´

Domain and the South Iberian paleomargin. Farther to the south in our

study areas, E-W trending folds are mainly north-vergent. In the

northeastern part of the cordillera, the IEZB has been interpreted as a

backthrust (Geel, 1973; Martín-Algarra, 1987; Jabaloy-S´

anchez et al.,

2007) with top-to-the-SSE/SE sense of movement (Lonergan et al.,

1994). Back-thrusts have also been recognized in the Flysch Complex,

Frontal Units and Mal´

aguide Complex (Foucault, 1976; García-Due˜

nas

and Navarro-Vil´

a, 1976; Rodríguez-Fern´

andez, 1982; Martín-Algarra,

1987; Balany´

a, 1991; Sanz de Galdeano et al., 1995; Geel and Roep,

1998; Fern´

andez-Fern´

andez et al., 2007; Martín-Algarra et al., 2009;

Jabaloy-S´

anchez et al., 2019a; Ruiz-Fuentes et al., 2022). The appear-

ance of S-vergent D

folds in the Alpuj´

arride Complex close to the IEZB

can therefore be interpreted in terms of propagation of deformation

associated to back-thrusting within the Albor´

an Domain acting as the

backstop. These observations allow us to correlate D

with the begin-

ning of the collision of Albor´

an with the External Domain, which

accounted around the late Burdigalian (e.g. Martín-Algarra, 1987;

Martín-Martín et al., 1996; Ruiz-Fuentes et al., 2022).

6. Conclusions

•Porphyroblasts in the Alpuj´

arride Complex preserve a succession of

three differently oriented FIA sets (WNW-ESE, E-W to NE-SW and

NNW-SSE) dened by sigmoidal or spiral-shaped inclusion trails. The

regional consistency of these microstructures and their preference

for subhorizontal or gentle plunges, support the view that FIAs

correspond to the intersections of steeply dipping and subhorizontal

foliations formed during alternating crustal shortening and gravita-

tional collapse (Bell and Johnson, 1989).

•The above implies that FIAs formed normal to crustal shortening

directions and have conserved their original orientations throughout

the subsequent orogenic evolution. This is shown to be consistent

with the Paleogene-Neogene history of relative plate-motions be-

tween Africa, Iberia and the Albor´

an Domain. A major change from

NNE-to NW-directed motion of Africa in the Oligocene (e.g. DeMets

et al., 2015) accounts for the two earliest formed FIA sets. The third

FIA is attributable to independent westward motion of the Albor´

Domain since the early Miocene.

•A strong predominance of inclusion-trail asymmetries (curvature

senses) associated with the rst two FIA sets indicating top-SSW to

top SE tectonic transport, favors a northward to NW-dipping sub-

duction zone accommodating Africa-Iberia convergence.

•Late-metamorphic structures (assigned to D

and D

events in pre-

vious works) deforming a composite S

foliation in the Alpuj´

arride

Complex represent at least four different generations: two associated

with steeply dipping crenulation cleavages striking broadly N–S and

ENE-WSW (D

), and two subhorizontal ones generated during

gravitational collapse, linked to andalusite growth (D

) and asso-

ciated with brittle-ductile shear bands and detachments (D

). These

structures bear witness of dynamic interactions between tectonic

stresses transmitted by Africa and the Albor´

an Domain and gravity.

Author statement

Alejandro Ruiz-Fuentes: conceptualization, methodology, investi-

gation, formal analysis, writing, visualization. Domingo Aerden:

conceptualization, methodology, writing.

A. Ruiz-Fuentes and D.G.A.M. Aerden

Journal of Structural Geology 168 (2023) 104823

Declaration of competing interest

The authors declare that they have no known competing nancial

interests or personal relationships that could have appeared to inuence

the work reported in this paper.

Data availability

Data will be made available on request.

Acknowledgements

We thank Etienne Skrzypek and two anonymous reviewers for

constructive comments that helped improve an early version of the

manuscript, and Toru Takeshita for his editorial work. We thank ´

Angel

Perandr´

es-Villegas for making the thin sections studied for this work and

F´

atima Linares Ord´

o˜

nez for the XCT scanning of samples. The research

was carried out as part of a Ph.D. project of ARF nanced by an FPU

grant from the Spanish Ministery of Education and Science, Culture and

Sports (FPU17/01874). Research expenses were covered by Spanish

government grant CGL2016-80687-R AEI/FEDER, and Junta de Anda-

lucía Projects P18-RT-3275 (AGORA), B-RNM-301-UGR18 (PAPEL) and

RNM148. Funding for open access charge: Universidad de Granada /

CBUA.

Appendix A. Supplementary data

Supplementary data to this article can be found online at https://doi.

org/10.1016/j.jsg.2023.104823.

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A. Ruiz-Fuentes and D.G.A.M. Aerden

Integrated Petrology, Geochronology and 3D Microstructural Analysis of Garnetiferous Micaschists: Insights Into the Tectono‐Metamorphic Evolution of the Betic Cordillera

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Apr 2025
J METAMORPH GEOL

Integration of microstructural, petrological and geochronological techniques has allowed detailed characterization of the timing and metamorphic features of deformation events in the Betic Cordillera. Phase equilibrium modelling, thermobarometric estimations, in situ U–Pb monazite geochronology and Ar–Ar geochronology (amphibole and micas) have been applied to key samples containing well‐constrained deformation fabrics and garnet porphyroblasts of different timing that can be distinguished based on their specifically orientated inclusion trails. Our study helps constrain the timing and kinematics of initial crustal thickening stages in the Alpujárride complex, followed by orogenic collapse and renewed continental collision around 17 Ma. In the underlying Nevado‐Filábride complex, multiple garnet generations coexisting in a single investigated sample record a similar history ending with garnet growth under low‐pressure/high‐temperature conditions around 13 Ma. New age constraints for microstructural trends fossilized within porphyroblasts refine the sequence of changing shortening directions previously linked to the plate‐motion interplay between the Alborán Domain, Iberia and Africa from Eocene to Middle Miocene.

Polyphase deformation of staurolite-bearing metapelite in the Chinese Altai: Recording a Devonian-Permian tectonic regime switch in the southwestern Central Asian Orogenic Belt

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Jul 2023
J STRUCT GEOL

Pre-Alpine prograde evolution of the Upper Alpujarride (Betic-Rif belt) reveals a Paleotethys-related collision

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Aug 2023

Structural controls on cobalt mineralisation during regional metamorphism: an example from the Rajapalot area, Finland

Article

Full-text available

Sep 2024
MINER DEPOSITA

The Rajapalot area of Finnish Lapland hosts an unusually high-grade association of cobalt-only and gold–cobalt deposits (10.91 Mt @ 2.5 g/t Au + 0.044% Co total inferred resources) within multiply folded metasedimentary rocks of the Paleoproterozoic Svecofennian collisional orogeny. Through the integration of X-ray computed micro-tomography and micro-X-ray fluorescence raster imaging of drill-core samples, we produce a model of cobalt-bearing ore mineralisation, which reveals primary fluid transportation mechanisms and precipitation pathways. When combined with the deposit-scale, high-resolution ground-based magnetic geophysical data, we show that cobalt-bearing ores in the Rajapalot region occur mostly as saddle reefs located in dilated fold hinges, which formed by simultaneous synthetic and antithetic shearing along developing crenulation-cleavage planes and incompetent bedding layers, respectively. We suggest that multi-layered rock complexes with alternations of competent and incompetent layers deformed and metamorphosed to upper greenschist-lower amphibolite facies should represent focus regions for cobalt exploration targeting campaigns in orogenic belts. The non-destructive workflow presented in this study could be an integral part of an early stage of cobalt mineral processing and traceability before metallurgical treatment.

Metamorphic evolution of Triassic rocks from the transition zone between the Maláguide and Alpujárride complexes (Betic Cordilleras, Spain)

Article

Full-text available

Oct 2005
EUR J MINERAL

A complete sequence of mineral assemblages ranging from late diagenesis to the epizone has been identified in Triassic rocks from “intermediate units” between the Alpujárride and the Maláguide complexes (Betic Cordillera, Spain). These units appear as a set of tectonic slices, the uppermost showing lithological characteristics similar to the Maláguide complex, which change as depth increases, towards lithologies typical of the Alpujárride complex. The study was carried out by X-ray diffraction, optical microscopy, electron microprobe and X-ray fluorescence. The mineralogical composition shows a clear vertical evolution: A dickite-bearing assemblage, characteristic of the upper slices is replaced by sudoite ± pyrophyllite-bearing assemblages, these by trioctahedral chlorite-rich assemblages, and finally by paragonite-bearing assemblages. Examination of bulk-rock chemistries indicates that most of the mineralogical changes are the result of the increasing metamorphic grade, with the exception of chloritoid; its growth does appear to be partly controlled by the rocks chemistry (it is Al2O3 dependent). The evolution of the metamorphic assemblages occurred prior to emplacement of the slices by thrusting and reveals a progressive transition in metamorphic grade between both complexes, although some discontinuities in the KI values between consecutive slices are observed.

Refined Tectonic Evolution of the Betic‐Rif Orogen Through Integrated 3‐D Microstructural Analysis and Sm‐Nd Dating of Garnet Porphyroblasts

Article

Full-text available

Oct 2022
TECTONICS

High‐resolution microstructural analysis of porphyroblast inclusion trails integrated with Sm‐Nd garnet geochronology has provided new insight into the tectonic history of the Betic‐Rif orogen. Three principal age groups of porphyroblasts are demonstrated with distinctly oriented inclusion‐trails. Inclusion‐trail curvature axes or “FIA” (Foliation Inflexion/Intersection Axes) are shown to represent “fossilized” crenulation axes from which a succession of different crustal shortening directions can be deduced. The regional consistency of microstructural orientations and their geometric relationship with multiple sets of macroscopic folds reveal the composite character of the Gibraltar Arc formed by a superposition of different folding directions and associated lineations. Bulk‐garnet ages of 35–22 Ma obtained from five micaschist samples of the Alpujarride‐Sebtide complex (ASC) and of 35–13 Ma from four micaschists of the Nevado‐Filabride complex (NFC) allow to deduce NNE‐SSW directed shortening in the Late Eocene changing to NW‐SE shortening in the early Oligocene, alternating with suborthogonal NE‐SW shortening during the Miocene. These directions can be related to a major swing in the direction of relative Africa‐Iberia plate‐motion known from kinematic modeling of magnetic seafloor anomalies, and subsequent dynamic interference between plate convergence and suborthogonal “tectonic escape” of the Alboran Domain. Coupled to previously established P‐T‐t paths, the new garnet ages support a common tectono‐metamorphic evolution of the ASC and NFC as laterally equivalent orogenic domains until, in the Miocene, the second became re‐buried under the first.

Tectonic Evolution of the Nevado‐Filábride Complex (Sierra de Los Filábres, Southeastern Spain): Insights From New Structural and Geochronological Data

Article

Full-text available

Aug 2022
TECTONICS

The high‐pressure metamorphic Nevado‐Filábride Complex (NFC) in the Betics mountain range of southeastern Spain exhibits continental and ocean‐derived tectonic units, which are key for understanding the geodynamic evolution of the Western Mediterranean. We address the current debate in the definition of tectonic units, the emplacement of (ultra)mafic rocks, and the timing of burial metamorphism by conducting a structural study combined with single grain fusion ⁴⁰Ar/³⁹Ar dating of white micas in structurally critical outcrops of the eastern Sierra de Los Filábres. One older ⁴⁰Ar/³⁹Ar age population (38–27 Ma) is found at distance from the main shear zones in the relics of an early foliation, while a younger ⁴⁰Ar/³⁹Ar population (22–12 Ma) is dominant in the vicinity of these shear zones, where the early foliation is obliterated. Both age groups are interpreted as the record of deformation or fluid‐induced recrystallization during distinct fabric‐forming events, while alternative scenarios are discussed. A key observation is the presence of an ophiolitic mélange, which—together with new and published geochronological data—allows for a new tectonic hypothesis. This considers Paleogene subduction beneath a Jurassic oceanic lithosphere, followed by the continued subduction of NFC and overlying ophiolites below the Alpujárride Complex. Exhumation during westward slab roll‐back led to the formation of an extensional detachment system that obliquely cut nappe contacts. Although the timing constraints for high pressure‐low temperature (HP‐LT) metamorphism in the NFC remain inconclusive, the new tectonic hypothesis provides a solution that can account for both Paleogene and Miocene ages of HP‐LT metamorphism.

Ar/Ar Age Constraints on HP/LT Metamorphism in Extensively Overprinted Units: The Example of the Alpujárride Subduction Complex (Betic Cordillera, Spain)

Article

Full-text available

Feb 2022
TECTONICS

Widespread overprinting of early high‐pressure/low‐temperature (HP/LT) subduction stages due to subsequent collisional or late‐orogenic tectono‐metamorphic events is a common feature affecting the interpretation of geochronologic data from HP/LT orogens. The Betic‐Rif orogen is exemplary in this connection as a great majority of published radiometric ages are found to cluster around 20 Ma. This clustering is commonly interpreted as reflecting a short, yet complex, succession of tectono‐metamorphic events spanning only over a few Myr, including back‐arc extension and overthrusting of the Internal Zones on the External Zones. An alternative explanation consists in the poor preservation of a much earlier HP/LT metamorphic event, presumably Eocene, coeval with subduction and crustal thickening in the Internal Zones, and particularly the Alpujárride Complex. However, this age is vividly debated due to widespread resetting by the Early Miocene HT/LP overprint. In this study, we provide new ⁴⁰Ar/³⁹Ar evidence from white micas selected along an E‐W section of the Internal Betics, from the central to the eastern Alpujárride Complex. Our new data show (a) that exceptionally well‐preserved HP/LT parageneses in this unit retain a well‐defined Eocene age around 38 Ma, and (b) that widespread 20 Ma ages recorded all along the section correspond to a regional stage of exhumation, coeval with a major change in the kinematics of back‐arc extension. Our study provides conclusive evidence that ⁴⁰Ar/³⁹Ar dating of carefully targeted HP/LT associations can overcome the problem of extensive late‐orogenic overprinting, testifying for an Eocene HP event around 38 Ma in the Betic‐Rif orogen.

Kinematics of subduction in the Ibero-Armorican arc constrained by 3D microstructural analysis of garnet and pseudomorphed lawsonite porphyroblasts from Île de Groix (Variscan belt)

Article

Full-text available

Apr 2021
SE

The small island of Groix in southern Brittany, France, is well known for exceptionally well-preserved outcrops of Variscan blueschists, eclogites, and garnetiferous mica schists that mark a Late Devonian suture between Gondwana and Armorica. The kinematics of polyphase deformation in these rocks is reconstructed based on 3D microstructural analysis of inclusion trails within garnet and pseudomorphed lawsonite porphyroblasts using differently oriented thin sections and X-ray tomography. Three sets of inclusion trails striking NE–SW, NNW–SSE, and WNW–ESE are recognized and interpreted to witness a succession of different crustal shortening directions orthogonal to these strikes. The curvature sense of sigmoidal and spiral-shaped inclusion trails of the youngest set is shown to be consistent with northwest and northward subduction of Gondwana under Armorica, provided that these microstructures developed by overgrowth of actively forming crenulations without much porphyroblast rotation. Strongly non-cylindrical folds locally found on the island are reinterpreted as fold-interference structures instead of having formed by progressive shearing and fold-axis reorientation. Six samples of a lower-grade footwall unit of the Groix ophiolitic nappe (Pouldu schists) were also studied. Inclusion trails in these rocks strike E–W, similar to the youngest set recognized on Groix island. They record Carboniferous N–S shortening during continental collision. These new microstructural data from southern Brittany bear a strong resemblance to earlier measured in inclusion-trail orientations in the northwestern Iberia Massif. A best fit between both regions suggests not more than about 15∘ anticlockwise rotation of Iberia during the Cretaceous opening of the Gulf of Biscay.

X-ray computed micro-tomography of spiral garnets: A new test of how they form

Article

Full-text available

Apr 2020
J STRUCT GEOL

Garnet porphyroblasts with spiral shaped inclusion trails of two micaschist samples have been investigated with X-ray computed tomography. For each garnet crystal, the orientation of its internal Foliation Intersection/Inflexion Axes (FIA) was measured, its volume and shape orientation, the latter via calculation of best-fit ellipsoids. Additionally, image analysis of inclusion trails was performed in slices passing through garnet centres oriented normal to FIA. Our data demonstrate subvertical and subhorizontal preferred orientations of the inclusion trails and of the porphyroblast crystals themselves in both samples. This supports a genetic model in which garnets periodically overgrew a succession of vertical and horizontal foliations during alternating crustal shortening and gravitational collapse without significantly rotating. The fact that both samples come from areas previously identified as regional shear zones supports the idea that porphyroblasts are able to maintain stable positions even in such environments. Our study exemplifies the strength of X-ray computed micro-tomography compared to previous techniques based on serial thin sections for distinguishing multiple FIA sets present in single samples.

Evolutionary geological models of the central-western peri-Mediterranean chains: a review

Article

Full-text available

Dec 2019

Two main groups of geological models presented over the last four decades on the palaeogeographic, paleotectonic and geodynamic eo-Alpine and neo-Alpine evolution of the central-western Mediterranean area were compared. The comparison was carried out mainly considering the main stratigraphic, sedimentological, petrographic, structural and plate tectonic constraints. Moreover, recent geophysical interpretations and reconstructions were also considered with an aim of presenting all the different results. The models can roughly be grouped into two main classes. First family considers the presence of the Mesozoic Tethyan Ocean, where a single oceanic basin is located between Africa and Europe and from which both eo-Alpine and neo-Alpine chains were generated during the Cretaceous to Miocene time span. Conversely, the other class considers the occurrence of at least two Tethyan oceanic branches (or with thinned continental crust) since the Jurassic, separated by one or more microcontinents. The pros and cons of both classes of models are presented. Progressive innovations and improvements to the two groups of models were proposed over the years. However, because the models are based on different data sets resulting from basic geological studies or obtained by means of other approaches, they often do not integrate easily. This caused interpretative difficulties and terminological uncertainties for their comparison, and completely different models were considered equivalent and, sometimes, the same terminology was used indifferently to identify different geological subjects. The main differences between the examined models concern the kinematic reconstructions and by hence in the palaeogeographic and paleotectonic interpretations. The discussion presented in this paper aims at contributing to clarify and update the state of knowledge on this controversial topic.

Lithological Successions of the Internal Zones and Flysch Trough Units of the Betic Chain

Chapter

Full-text available

Jun 2019

Mesozoic lithological successions of the Alborán domain record two rifting events. An initial Triassic rifting stage is characterized by thick stratigraphic successions showing typical Alpine facies, both siliciclastic and carbonatic, respectively deposited in continental to coastal and shallow to moderately deep marine environments. A second rifting event took place during the Jurassic and culminated in the diachronous opening of the so-called Betic-Maghrebian Ocean, a part of the Tethys realm. The Cretaceous successions of the Alborán domain record the post-rift evolution of the proximal to distal parts of a divergent Tethyan paleomargin, while those of the Campo de Gibraltar Flysch Complex record the evolution of the oceanic basin. All these domains were later transformed into a convergent continental margin (Oligocene to Early Miocene) that later evolved to a collisional setting (Middle to Late Miocene).

Structural record of polyorogenic pre-Alpine and Alpine deformations within a major thrust nappe close to a suture zone (Internal-External Zones Boundary of the central Betic Cordillera, S Spain)

Article

Oct 2022

The Malaguide Complex occupies the highest position in a thick-skinned Alpine thrust stack. It includes polydeformed, sedimentary and low-grade metamorphic rocks of Palaeozoic basements and their Meso-Cenozoic covers. NE of Granada the complex results severely sheared and stretched along the Betic suture zone in the metamorphic hinterland (orogenic backstop). The suture developed after the Burdigalian, due to oblique collision of the Alboran Domain (including the Malaguide Complex) with the South-Eastern Iberian Paleomargin. The orogenic evolution in the backstop region started with late Palaeogene to Early Miocene N-directed pre-collisional shortening and early thrust stacking, associated with low-grade metamorphism in the lowest Malaguide units increasing downwards into the underlying Alpujarride Complex. An accretionary prism developed at the expense of deep-marine (Flysch) sediments and the sedimentary covers of the Iberian and Alboran conjugate margins (External Zones and Frontal units, respectively). Later orogenic stages were characterized by SSE-directed back-folding and -thrusting in the Middle Miocene affecting both sides of the Betic suture related to dextral wrenching causing plurikilometric displacements. In this way, the suture evolved towards a several kilometres wide megashear zone (Cogollos-Diezma Corridor), prior to a final phase of SW-directed post-orogenic extension in the Late Miocene. Despite this protracted Alpine evolution, the Malaguide basements also preserve a record of orogenic activity during Late Carboniferous stages of the Variscan Orogeny in the form of a set of NE-SW trending folds with SE vergence and associated slaty cleavage, which are not found in the post-Carboniferous cover. The temperature conditions of this event (>300°C) in the less metamorphic Malaguide units are determined by colour alteration index (CAI) data from conodonts that have also allowed biostratigraphic dating of the Devonian-Carboniferous successions.

Alpine Orogeny: Deformation and Structure in the Southern Iberian Margin (Betics s.l.)

Chapter

Jun 2019

Closure of the Tethys Ocean during the Neogene brought the Alborán Domain allochthonous terrane to collide with the paleomargins of both NW Africa and S Iberia. During the collision, parts of the South Iberian Domain were deformed by a thin-skinned thrust-and-fold system, while another part (Nevado-Filábride Complex) was subducted below the Alborán Domain and accreted below it after undergoing Alpine HP-LT metamorphism and ductile deformations. The Flysch Trough units are composed of deep marine sediments that were deformed by thin-skinned thrust-and-fold systems and thrust over the South Iberian Domain. Below the sea level, the Betic-Rif orogenic system extends westwards into the Gulf of Cádiz area where several orogenic wedges are thrust over the oceanic crust of both the African and the European plates.

Deciphering western Mediterranean kinematics using metamorphic porphyroblasts from the Alpujárride Complex (Betic Cordillera)

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